In general, composite ceramic powder containing two or more types of ceramic materials is constituted of secondary particles or composite particulates each of which is formed by agglomerating primary particles made of respective ceramic materials. A conventional method of manufacturing such composite ceramic powder includes the steps of providing two or more types of coarse powdered materials, pulverizing the coarse powdered materials by using a pulverizer to form fine powdered materials, mixing the produced fine powdered materials in a ball mill, calcining the mixture, and milling the calcined mixture to form composite ceramic particulates. Another conventional method of manufacturing such composite ceramic powder includes the steps of dissolving two or more types of powdered materials to form a solution of the materials, thermally decomposing by dropping the solution into a furnace, calcining the decomposed materials, and milling the calcined materials to form fine composite ceramic particulates. The latter method is known as a drip pyrolysis.
Some composite ceramic powder thus manufactured has been used as a material for forming electrodes of an SOFC.
In the composite ceramic powder thus manufactured, the composite ceramic particulates have irregular shapes. This is because they are produced by mechanically grinding mass products which are formed by agglomerating the primary particles made of respective materials. Thus, it is substantially impossible to obtain the composite ceramic particulates having spherical shapes. Moreover, the electrodes of the SOFC made of such composite ceramic powder do not exhibit acceptable electrical properties.
The present inventors have developed composite ceramic powder constituted of spherical composite particulates and a method of manufacturing the same. The developed technique is fully shown in Japanese Patent Application No. 6-82399 (Japanese Laid-Open Patent Publication No. 7-267613) previously filed.
As described therein, the composite ceramic particulates contained in the composite ceramic powder are solid spherical particulates each of which is produced by agglomerating the primary particles in a substantially uniform dispersion state. Further, the method of manufacturing the composite ceramic powder constituted of such spherical composite particulates includes the steps of atomizing a solution of two or more types of raw materials to form mist thereof, drying the mist at temperatures below the temperature at which the raw materials can be thermally decomposed, and then thermally decomposing the raw materials.
This technique may provide the composite ceramic powder containing the spherical composite particulates each of which is produced by agglomerating the primary particles in the substantially uniform dispersion state. The composite ceramic powder thus manufactured may exhibit increased electrical properties. Therefore, the composite ceramic powder is very suitable as the materials for forming the electrodes.
In the composite ceramic powder thus manufactured, the primary particles in each spherical composite particulate are substantially uniformly dispersed. In other words, each composite particulate has a uniform composite form. However, if each type of primary particles may exhibit inherent functions, it is not necessarily desirable that the composite particulate has such a uniform composite form.
It is desired to develop a new and superior composite form of the secondary particles or composite ceramic particulates in which the primary particles are preferably dispersed in a controlled dispersion state, thereby further increasing properties of composite ceramic powder constituted of such composite ceramic particulates and articles made of the composite ceramic powder and finding new uses of the composite ceramic powder.
Moreover, the electrodes of the SOFC formed of the conventional composite ceramic powder gradually deteriorates with time and results in inferior performance. Such deterioration of the electrodes is caused by undesirable aggregation of the conductive materials derived from some of the primary particles. In general, the electrodes of the SOFC must maintain their performance for a long period of time such that the SOFC can be continuously used over tens of thousands of hours.
To remove such a disadvantage, it is desired to change the composite form of the composite ceramic particulates of the composite ceramic powder for effectively preventing the undesirable aggregation of the primary particles, so that the electrodes made of the composite ceramic powder do not degrade for a long period of time.